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The prebiotic molecular inventory of Serpens SMM1 I. An investigation of the isomers CH$_{3}$NCO and HOCH$_{2}$CN

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Methyl isocyanate (CH$_{3}$NCO) and glycolonitrile (HOCH$_{2}$CN) are isomers and prebiotic molecules that are involved in the formation of peptide structures and the nucleobase adenine, respectively. ALMA observations of the intermediate-mass Class 0 protostar Serpens SMM1-a and ALMA-PILS data of the low-mass Class 0 protostar IRAS~16293B are used. Spectra are analysed with the CASSIS line analysis software package in order to identify and characterise molecules. CH$_{3}$NCO, HOCH$_{2}$CN, and various other molecules are detected towards SMM1-a. HOCH$_{2}$CN is identified in the PILS data towards IRAS~16293B in a spectrum extracted at a half-beam offset position from the peak continuum. CH$_{3}$NCO and HOCH$_{2}$CN are equally abundant in SMM1-a at [X]/[CH$_{3}$OH] of 5.3$\times$10$^{-4}$ and 6.2$\times$10$^{-4}$, respectively. A comparison between SMM1-a and IRAS~16293B shows that HOCH$_{2}$CN and HNCO are more abundant in the former source, but CH$_{3}$NCO abundances do not differ significantly. Data from other sources are used to show that the [CH$_{3}$NCO]/[HNCO] ratio is similar in all these sources within $\sim$10\%. The new detections of CH$_{3}$NCO and HOCH$_{2}$CN are additional evidence for a large interstellar reservoir of prebiotic molecules that can contribute to the formation of biomolecules on terrestrial planets. A plausible formation pathway for HOCH$_{2}$CN is the thermal Strecker-like reaction of CN$^{-}$ with H$_{2}$CO. The similar [CH$_{3}$NCO]/[HNCO] ratios indicate that these two species either are chemically related or their formation is affected by physical conditions in the same way. The relatively high abundances of HOCH$_{2}$CN and HNCO in SMM1-a may be explained by a prolonged stage of relatively warm ice mantles, where thermal and energetic processing of HCN in the ice results in the efficient formation of both species.
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Astronomy &Astrophysics manuscript no. SMM1_C2H3NO_isomers ©ESO 2021
January 1, 2021
The prebiotic molecular inventory of Serpens SMM1
I. An investigation of the isomers CH3NCO and HOCH2CN
N.F.W. Ligterink1, A. Ahmadi2, A. Coutens3, Ł. Tychoniec2H. Calcutt4,5, E.F. van Dishoeck2,6, H. Linnartz7, J.K.
Jørgensen8, R.T. Garrod9, and J. Bouwman7
1Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: niels.ligterink@csh.unibe.ch
2Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
3Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Georoy Saint-Hilaire, 33615 Pessac, France
4Department of Space, Earth and Environment, Chalmers University of Technology, 41296, Gothenburg, Sweden
5Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100
Torun, Poland
6Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
7Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
8Centre for Star and Planet Formation, Niels Bohr Institute & Natural History Museum of Denmark, University of Copenhagen,
Øster Voldgade 5–7, 1350 Copenhagen K., Denmark
9Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA
Received October 8, 2020; accepted December 22, 2020
ABSTRACT
Aims. Methyl isocyanate (CH3NCO) and glycolonitrile (HOCH2CN) are isomers and prebiotic molecules that are involved in the
formation of peptide structures and the nucleobase adenine, respectively. These two species are investigated to study the interstellar
chemistry of cyanides (CN) and isocyanates (NCO) and to gain insight into the reservoir of interstellar prebiotic molecules.
Methods. ALMA observations of the intermediate-mass Class 0 protostar Serpens SMM1-a and ALMA-PILS data of the low-mass
Class 0 protostar IRAS 16293B are used. Spectra are analysed with the CASSIS line analysis software package in order to identify
and characterise molecules.
Results. CH3NCO, HOCH2CN, and various other molecules are detected towards SMM1-a. HOCH2CN is identified in the PILS data
towards IRAS 16293B in a spectrum extracted at a half-beam oset position from the peak continuum. CH3NCO and HOCH2CN
are equally abundant in SMM1-a at [X]/[CH3OH] of 5.3×104and 6.2×104, respectively. A comparison between SMM1-a and
IRAS 16293B shows that HOCH2CN and HNCO are more abundant in the former source, but CH3NCO abundances do not dier
significantly. Data from other sources are used to show that the [CH3NCO]/[HNCO] ratio is similar in all these sources within 10%.
Conclusions. The new detections of CH3NCO and HOCH2CN are additional evidence for a large interstellar reservoir of prebiotic
molecules that can contribute to the formation of biomolecules on terrestrial planets. The equal abundances of these molecules in
SMM1-a indicate that their formation is driven by kinetic processes instead of thermodynamic equilibrium, which would drive the
chemistry to one product. HOCH2CN is found to be much more abundant in SMM1-a than in IRAS 16293B. From the observational
data, it is dicult to indicate a formation pathway for HOCH2CN, but the thermal Strecker-like reaction of CNwith H2CO is a
possibility. The similar [CH3NCO]/[HNCO] ratios found in the available sample of studied interstellar sources indicate that these
two species either are chemically related or their formation is aected by physical conditions in the same way. Both species likely
form early during star-formation, presumably via ice mantle reactions taking place in the dark cloud or when ice mantles are being
heated in the hot core. The relatively high abundances of HOCH2CN and HNCO in SMM1-a may be explained by a prolonged stage
of relatively warm ice mantles, where thermal and energetic processing of HCN in the ice results in the ecient formation of both
species.
Key words. Astrochemistry – Astrobiology – Individual Objects: Serpens SMM1 – ISM: abundances – Submillimeter: ISM
1. Introduction
Observations of molecules towards star-forming regions give in-
sight into the kind of species that end up in planet-forming discs.
These molecules not only aid planet formation but can also seed
newly formed planets with a cocktail of molecules from which
larger organic molecules can be formed. Prebiotic molecules are
of particular interest, as they are involved in the formation of
biomolecules, such as amino acids, nucleobases, proteins, and
lipids (Sandford et al. 2020). In the interstellar medium (ISM)
and on planets, prebiotic molecules are the building blocks from
which biomolecules are made.
In the ISM, several prebiotic molecules have been de-
tected. Examples are formamide (NH2CHO, Rubin et al.
1971) a precursor to nucleobases and amino acids (Saladino
et al. 2012), the simplest sugar-like molecule glycolaldehyde
(HOCH2CHO, Hollis et al. 2000;Jørgensen et al. 2012), methy-
lamine (CH3NH2,Kaifu et al. 1974;Bøgelund et al. 2019) and
aminoacetonitrile (NH2CH2CN, Belloche et al. 2008), building
blocks of the amino acid glycine (Holtom et al. 2005;Lee et al.
2009), the peptide building blocks acetamide (CH3C(O)NH2)
Article number, page 1 of 32
arXiv:2012.15672v1 [astro-ph.SR] 31 Dec 2020
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Methyl isocyanate Glycolonitrile
Fig. (1) Structures of the C2H3NO isomers methyl isocyanate
(CH3NCO, left) and glycolonitrile (HOCH2CN, right).
and N-methylformamide (CH3NHCHO, Hollis et al. 2006;
Halfen et al. 2011;Belloche et al. 2017,2019;Ligterink et al.
2020), the chiral molecule propylene oxide (CH3CHCH2O,
McGuire et al. 2016), cyanomethanimine (NHCHCN), which
can oligomerise to form adenine (Zaleski et al. 2013;Riv-
illa et al. 2019), and the nucleobase precursors cyanamide
(NH2CN, Turner et al. 1975;Coutens et al. 2018), hydroxy-
lamine (NH2OH, Rivilla et al. 2020) and carbamide (also known
as urea, NH2C(O)NH2,Belloche et al. 2019). Over the past
years, methyl isocyanate (CH3NCO, Halfen et al. 2015;Cer-
nicharo et al. 2016;Ligterink et al. 2017) has been detected
in several interstellar sources and recently its isomer glycoloni-
trile (also known as hydroxy acetonitrile, HOCH2CN, Zeng
et al. 2019) was identified for the first time in the ISM to-
wards the low-mass protostar IRAS 16293–2422B (hereafter
IRAS 16293B). Both these isomers are prebiotic molecules.
CH3NCO can engage in reactions that form peptide-like struc-
tures, while HOCH2CN is known to accelerate the oligomeri-
sation of HCN in liquids and ice under terrestrial conditions,
forming the nucleobase adenine (Schwartz & Goverde 1982;
Schwartz et al. 1982).
Besides their relevance to prebiotic chemistry, CH3NCO and
HOCH2CN are also interesting molecules to gain insight into in-
terstellar nitrogen chemistry. While these molecules, being iso-
mers, have the same elemental composition (C2H3NO), their
chemical structures dier significantly, see Fig. 1. Recent quan-
tum chemical calculations of the stability of C2H3NO isomers
in general, also reveal that CH3NCO is the most stable species
of those, followed by HOCH2CN (Fourré et al. 2020). There-
fore, observations of this isomer couple provide information on
interstellar reactions involving cyanides (-CN) and isocyanates
(-NCO), two important nitrogen-bearing chemical groups, and
the physical conditions that steer or prohibit this chemistry.
After the first detection of CH3NCO (Halfen et al. 2015;Cer-
nicharo et al. 2016), HNCO was suggested to be involved in its
formation due to their structural similarity and the large HNCO
abundances in interstellar gas and ice (in the form of OCN
Boogert et al. 2015). Various gas-phase and solid-state methy-
lation (the addition of a CH3functional group to a molecule)
reactions of HNCO, the OCN radical, and the OCNanion have
been proposed as possible reaction pathways. Experimental in-
vestigations (Ligterink et al. 2017;Maté et al. 2018) and model-
ing studies (Martín-Doménech et al. 2017;Quénard et al. 2018;
Majumdar et al. 2018) indicate solid-state methylation in the ice
mantle as the main pathway to form CH3NCO:
CH3+NCO CH3NCO ·(1)
Variations of this pathway, such as the methylation of HNCO
or OCNmay be possible as well, while completely dierent
reactions, such as the hydrogenation of HCN. . .CO may form
CH3NCO as well.
No gas-phase formation pathways are known for glycoloni-
trile, but solid-state production routes have been studied theo-
retically (Woon 2001) and experimentally (Danger et al. 2012,
2014). Laboratory work indicates that the thermally activated
reaction between a cyanide anion (CN) and formaldehyde
(H2CO) forms HOCH2CN:
[XH+CN]+H2CO HOCH2CN +X,(2)
where X is a molecule that can act as a base, such as am-
monia (NH3) or water (H2O). This reaction is the solid-state
equivalent of the Strecker synthesis, which is a sequence of
chemical reactions that produce amino acids. The Strecker-
like formation of HOCH2CN is linked to the formation of
aminomethanol (HOCH2NH2,Bossa et al. 2009) and aminoace-
tonitrile (NH2CH2CN, Danger et al. 2011). The latter of these
species is detected in the ISM and known as a possible inter-
mediate in the formation of the amino acid glycine (Belloche
et al. 2008). Irradiation of HOCH2CN results in the photoprod-
ucts formylcyanide (HC(O)CN) and ketenimine (CH2CNH). Al-
though not investigated, hydrogenation and oxygen additions of
these two species may provide pathways to form HOCH2CN in
the ISM. The solid-state radical-radical reactions HO +CH2CN
and HOCH2+CN can also form glycolonitrile, but neither of
these reactions has been investigated (Margulès et al. 2017).
However, precursor species to these reactions can be present in
ice mantles, in particular when methanol (CH3OH) or acetoni-
trile (CH3CN) are energetically processed (Allamandola et al.
1988;Hudson & Moore 2004;Bulak et al. 2020).
Methyl isocyanate and glycolonitrile can thus be used as
tracers of reactions involving CN and NCO and investigating
their interstellar abundances reveals information about the chem-
ical and physical processes that drive these reactions and inter-
stellar nitrogen chemistry as a whole. CH3NCO and HOCH2CN
have only been detected simultaneously in the low-mass pro-
tostar IRAS 16293B (Ligterink et al. 2017;Martín-Doménech
et al. 2017;Zeng et al. 2019), albeit in dierent observational
data sets. Due to this limited sample size, it is dicult to de-
rive correlations or variations in the C2H3NO isomer chemistry
and therefore simultaneous identifications in other sources are
required. Here, deep ALMA observations of the intermediate-
mass Class 0 protostar Serpens SMM1-a (hereafter SMM1-a)
are presented to derive additional constraints on CH3NCO and
HOCH2CN chemistry.
The Serpens star-forming region contains multiple deeply
embedded sources, of which SMM1 is the brightest (Casali et al.
1993). The Serpens region contains multiple outflows and jets,
some of which originate from SMM1 (Dionatos et al. 2013;Hull
et al. 2016). The chemistry of the SMM1 hot corino, its out-
flows, and the Serpens core have been characterised in various
studies (e.g., White et al. 1995;Hogerheijde et al. 1999;Kris-
tensen et al. 2010;Öberg et al. 2011;Goicoechea et al. 2012;Ty-
choniec et al. 2019). High-resolution continuum jet observations
have shown that SMM1 consists of multiple sources, of which
SMM1-a is the main one (Choi 2009;Dionatos et al. 2014;Hull
et al. 2017). SMM1-a has SMM1-b as a close neighbor at 500
au, while two other sources, SMM1-c and -d, are located fur-
ther away to its north. Recent distance measurements place the
Serpens core, and therefore SMM1, at a distance of 436.0±10
pc (Ortiz-León et al. 2017), resulting in a luminosity estimate of
the entire SMM1 source of 100 L. SMM1-a is considered to
Article number, page 2 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
be an intermediate-mass protostar (Hull et al. 2017;Tychoniec
et al. 2019).
In this work, the detection and analysis of the isomers
HOCH2CN and CH3NCO towards SMM1-a are presented and
compared with literature results of IRAS 16293B and those of
other sources. In section 2the observations towards SMM1 and
the analysis method are presented. The detections of HOCH2CN,
CH3NCO, and various other molecules are presented in section
3. Section 4discusses these detections and their likely formation
pathways. The conclusions of this work are presented in section
5.
2. Data & Methods
2.1. Observations and spectra of Serpens SMM1
SMM1 was observed on 27-March-2019 during ALMA cycle 6,
as part of project #2018.1.00836.S (PI: N.F.W. Ligterink). The
region was observed using a total of 42 antennae with baselines
spanning 15 – 332 meters in configuration C43-5. The on-source
integration time was 50 minutes, towards the phase centre αJ2000
=18:29:49.80 δJ2000 = +01:15:20.6. Spectra were recorded in
select frequency windows between 217.59 and 235.93 GHz, at
resolutions of 488.21 kHz (0.33 km s2) and 1952.84 kHz (1.25
km s2) for the continuum window, see Table 1. The data were
calibrated and imaged with version 5.4.0-70 of the Common
Astronomy Software Applications (CASA). Bandpass and flux
calibration was conducted on J2000–1748, while phase calibra-
tion was performed on J1851+0035. The flux uncertainty was
20%. To reach the desired sensitivity, the measurement sets
were cleaned using the Hogbom algorithm (Högbom 1974) and
Briggs weighting with a robust parameter of 0.5. This resulted
in an angular resolution of 100
.32×100
.04 and an rms noise of 2.6
mJy beam1km s1in the final spectral data cubes. The primary
beam of the observations was 2600.
Table (1) Frequency settings of ALMA SMM1 observations
Frequency range Bandwidth Resolution
(GHz) (GHz) (kHz) (km s1)
217.59 – 217.70 0.117 488.21 0.33
217.97 – 218.09 0.117 488.21 0.33
218.43 – 218.55 0.117 488.21 0.33
218.92 – 219.03 0.117 488.21 0.33
219.71 – 219.82 0.117 488.21 0.33
221.30 – 221.42 0.117 488.21 0.33
221.42 – 221.53 0.117 488.21 0.33
221.53 – 221.65 0.117 488.21 0.33
231.74 – 231.97 0.234 488.21 0.33
233.41 – 233.65 0.234 488.21 0.33
234.06 – 235.93 1.875 1952.84 1.25
Due to the line-richness of the source, the following pro-
cedure was followed to properly subtract the continuum from
the line observations. We imaged all spectral windows without
the continuum removed and used the corrected sigma-clipping
method of the STATCONT package1(Sánchez-Monge et al.
2018) to extract a continuum-subtracted line cube. STATCONT
can only subtract zeroth-order polynomials, while in this dataset
non-zeroth-order baselines are visible. Furthermore, continuum
subtraction in the uv-plane is more desirable since the deconvo-
lution of the line emission is more robust when it is not subjected
1https://hera.ph1.uni-koeln.de/~sanchez/statcont
to the deconvolution errors of the brighter continuum. There-
fore, the STATCONT outputs were used to identify the line-free
channels in the spectra and the continuum was subtracted in the
uv-plane with the uvcontsub task in CASA. Line-free channels
are sparse, but for most spectral windows, at least 20% of the
bandwidth was given as input to the uvcontsub task, with the ex-
ception of two spectral windows, where only 10% of the band-
width was line-free. From the resulting datacube, the SMM1-
a hot core spectrum was extracted towards the peak continuum
position αJ2000 =18:29:49.793, δJ2000 = +1.15.20.200. From the
average continuum flux density (0.41 mJy beam1), the back-
ground temperature was determined to be 5.2 K.
2.2. PILS observations and spectra of IRAS 16293B
In this work, the column density of HOCH2CN is determined in-
dependently from the detection presented by (Zeng et al. 2019)
by analysing data from the Protostellar Interferometric Line Sur-
vey (PILS). Other species relevant to this work are also searched
for in the PILS data set. The observational details of the PILS
survey have been presented in various other publications (e.g.,
Jørgensen et al. 2016) and here only the most relevant informa-
tion is presented. In short, the PILS survey makes use of ALMA
band 7 observations, covering a frequency range from 329 to
363 GHz at a spatial resolution of 000
.5. To investigate the chem-
ical inventory of IRAS 16293B, spectra are extracted at several
positions. These positions are on the peak continuum, a half-
beam oset from the peak continuum, and a full-beam oset
from the peak continuum. Most PILS analyses of IRAS 16293B
make use of the spectrum at the full-beam oset position Jør-
gensen et al. (e.g., 2016); Coutens et al. (e.g., 2016); Ligterink
et al. (e.g., 2017); Coutens et al. (e.g., 2018); Persson et al. (e.g.,
2018); Calcutt et al. (e.g., 2018); Jørgensen et al. (e.g., 2018). In
this work, this is the main position for which molecular ratios
with HOCH2CN are determined, but the spectra of other posi-
tions are also analysed. The systemic velocity towards these po-
sitions is VLSR =2.7 km s1and the line width is approximately
V=1.0 km s1. Due to the dense dust around IRAS 16293B,
the background temperatures (TBG) at these positions are higher
than the cosmic microwave background radiation temperature of
2.7 K and couple with the molecular line emission (see Ligterink
et al. 2018). At the full-beam oset position TBG =21 K, while
at the half-beam oset position it is TBG =52 K.
2.3. Analysis method
The spectra were analyzed with the CASSIS2line analysis soft-
ware. Spectral line lists were obtained from the JPL database
for molecular spectroscopy (Pickett et al. 1998), the Cologne
Database for Molecular Spectroscopy (CDMS, Müller et al.
2001,2005), and from literature. An overview of the spectro-
scopic line lists used in this work and the laboratory works they
are based on is given in Appendix A. Given a spectroscopic
line list as input, CASSIS can produce synthetic spectra of a
molecule based on parameters such as column density (NT), ex-
citation temperature (Tex), peak gas velocity (VLSR), line width at
half maximum (V), and source size (θsource). These parameters
were given as free parameters to a Monte-Carlo Markov Chain
(MCMC) algorithm and χ2minimisation routine. This routine
finds the best-fit of a synthetic spectrum to an observed spec-
trum over a specified parameter space, thereby determining the
2CASSIS has been developed by IRAP-UPS/CNRS
(http://cassis.irap.omp.eu)
Article number, page 3 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Fig. (2) Moment 0 maps of the HOCH2CN 248,17/16–238,16/15 , Eup =222 K line and the CH3NCO ν=0 250,0–240,0, Eup =188 K
line towards SMM1. Both lines are integrated over 8 velocity bins, centered on the peak frequency of each line as determined
towards SMM1-a. Positions of protostars in the SMM1 region are indicated and the beam size (100
.32×100
.04) is visualised in the
bottom left corner. Dust continuum contours are given by the black dotted line at the levels of 0.02, 0.05, 0.1, 0.2, 0.5 Jy Beam1.
Table (2) Best-fit parameters for molecules detected towards SMM1-a in a 100
.2 beam.
Molecule Lines NTTex VLSR V[X] /[CH3OH] [X] /[HNCO]
# (cm2) (K) km s1km s1
HOCH2CN 18 (7.4±0.9)×1014 260±45 6.8±0.2 2.5±0.3 6.2×1047.4×102
CH3NCO 12 (6.4±1.9)×1014 240±60 7.0±0.4 3.0±0.6 5.3×1046.4×102
D2CO 3 (5.4±2.5)×1014 [200] 7.4±0.4 3.9±0.2 4.5×1045.4×102
CH18
3OH 4 (2.0±1.1)×1015 250±60 7.1±0.2 2.8±0.3 –
12CH3OHa5b1.1×1018 – – – 1.0 110
CH3CNc6 (1.3±0.3)×1015 190±25 7.5±0.2 3.4±0.3 1.1×1030.1
NH2CN 3 (5.1±1.3)×1013 190±40 7.1±0.2 3.2±0.4 4.3×1055.1×103
HN13CO 3 (1.9±0.3)×1014 190±30 7.6±0.2 3.5±0.3 –
HN12COa5b1.0×1016 – – – 1.1×1021.0
CH3CH2OH 14 (4.1±0.9)×1015 210±25 7.3±0.2 2.8±0.6 4.1×1030.3
CH3OCHO 24 (7.4±0.7)×1015 215±20 7.3±0.2 3.1±0.3 7.4×1030.6
a-(CH2OH)214 (1.7±0.5)×1015 195±70 7.2±0.2 2.7±0.3 1.7×1030.1
CH3CNO 0 1.0×1013 [200] [7.0] [3.5] 9.1×1061.0×103
CH3OCN 0 5.0×1013 [200] [7.0] [3.5] 4.6×1065.0×103
CH2CNH 0 1.0×1015 [200] [7.0] [3.5] 8.3×1040.1
CH(O)CN 0 2.0×1014 [200] [7.0] [3.5] 1.7×1042.0×102
NH2CH2CN 0 1.0×1014 [200] [7.0] [3.5] 8.3×1051.0×102
Notes. Values in brackets are assumed. aMain isotopologue column densities are determined by applying the ratios 16O/18O=560 and 12C/13C=
52.5 to the minor isotopologue column densities. bThe number of lines identified of the main isotopologue in this data set. cThe CH3CN best-fit
parameters are determined from its vibrationally excited state ν8=1.
best-fit parameters and thus column density and excitation tem-
perature of a molecule. For the analysis, optically thin lines (τ
1.0) were used. The τ-value was approximated from the by-eye
synthetic fit (see below) of the observed rotational lines with the
CASSIS software. The molecules were assumed to be in local
thermodynamic equilibrium (LTE). Errors on physical parame-
ters take the uncertainty of the fit and the flux uncertainty as
input and are calculated from the spread in χ2values around the
minimum to a 3σconfidence level.
In this work, spectral lines of a molecule were identified in
the observed spectra and a by-eye synthetic fit of the lines was
made. For the by-eye fit, line width and peak velocity are de-
termined from prominent spectral lines of molecules such as
HNCO and CH3OH and used as a first approximation for other
molecules. An excitation temperature of 200 K is taken as an
initial guess and followed by a round of adjusting column den-
sity and rotational temperature until a reasonable by-eye fit was
found. The by-eye fit results were given as starting parameters
for the MCMC χ2minimisation routine. The χ2minimisation
was performed on lines that are not blended and have mini-
mal contributions from the wings of neighboring lines. Blending
species were identified by checking the line position for other
lines of known hot core /corino species with Aij >1×106and
Eup of 0–1000 K. The column density was given as a free param-
eter over two orders of magnitude centered on the by-eye fit col-
umn density and the excitation temperature was a free parameter
Article number, page 4 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Table (3) Best-fit parameters of HOCH2CN and related species towards IRAS 16923B in the PILS data set in a 000
.5 beam
Molecule NTTex [X] /[CH3OH]a[X] /[HNCO]a
(cm2) (K)
HOCH2CN 1.0×1015 [150] 1.0×1043.3×102
HOCH2CN 1.0×1015 [300] 1.0×1043.3×102
CH2CNH 1.0×1015 [100] 1.0×1043.3×102
CH2CNH 2.0×1015 [300] 2.0×1046.7×102
CH(O)CN 5.0×1014 [100] 5.0×1051.7×102
CH(O)CN 5.0×1014 [300] 5.0×1051.7×102
NH2CH2CN 1.0×1015 [100] 1.0×1043.3×102
NH2CH2CN 5.0×1014 [300] 5.0×1051.7×102
Notes. Values in brackets are assumed. VLSR =2.7 km s1and V=1.0 km s1. Note that some upper limit column densities are similar because
the upper state energies of the lines cover only a narrow range of energies. aCH3OH and HNCO column densities were adopted from Jørgensen
et al. (2018) and Coutens et al. (2016), respectively.
Fig. (3) Moment 0 map of the HOCH2CN 364,32–354,31 , Eup =
324 K line towards IRAS 16293B. The line is integrated over
8 velocity bins, centered on the peak frequency of the line. The
positions of the half-beam and full-beam oset positions around
IRAS 16293B are indicated and the beam size (000
.5×000
.5) is vi-
sualised in the bottom left corner. Dust continuum contours are
given by the black dotted line at the levels of 0.02, 0.05, 0.1, 0.2,
0.5 Jy Beam1.
from 50 – 350 K. For SMM1-a, Vwas a free parameter from
1.0 – 4.0 km s1, and the source velocity was a free parameter
between 6.0 – 9.0 km s1. The source size was assumed to be
equal to the beam size and taken to be 100
.2, resulting in a beam
filling factor of 0.5. A background continuum temperature (Tbg)
of 5.2 K was used.
For the analysis of IRAS 16293B, Vand VLSR were fixed
to 1.0 km s1and 2.7 km s1, respectively. The source size was
assumed to be equal to the beam size at 000
.5, while a background
continuum temperature of Tbg =21 K was used.
3. Results
3.1. SMM1
In the spectra of SMM1-a, multiple unblended rotational lines
of the isomers HOCH2CN and CH3NCO ν=0 are found. Mo-
ment 0 maps (the spatial mapping of the integrated line in-
tensity of a single rotational line) of both species show that
most emission originates from SMM1-a, see Fig. 2. The iden-
tified lines towards SMM1-a are presented in Figs. 4and 5. For
HOCH2CN, this is the second independent interstellar detection
of this molecule (the first detection of HOCH2CN was presented
towards IRAS 16293B by Zeng et al. 2019), while the first de-
tection of CH3NCO towards SMM1-a is part of only a handful
of detections of this species towards other interstellar sources.
Several other species are identified in the SMM1-a spec-
tra as well. Rotational lines of acetonitrile (CH3CN ν8=1),
cyanamide (NH2CN), ethanol (CH3CH2OH), the anti-conformer
of ethylene glycol (a-(CH2OH)2), deuterated formaldehyde
(D2CO), isocyanic acid (HN12CO and HN13CO), methylformate
(CH3OCHO), and methanol (12CH3OH and CH18
3OH) are de-
tected. Rotational lines of the C2H3NO isomers methyl fulmiate
(CH3CNO) and methyl cyanate (CH3OCN) were not identified
in the spectra. The molecules aminoacetonitrile (NH2CH2CN),
ketenimine (CH2CNH), and formylcyanide (HC(O)CN) are
searched for, but not identified. Spectra are presented in Ap-
pendix Band spectroscopic parameters of transitions are pro-
vided in Table 5. Following the procedure detailed in Sect. 2.3,
the best-fit parameters of these species are determined. For un-
detected species, upper limit column densities are determined by
assuming Tex =200 K, which is chosen as a representative exci-
tation temperature from the molecules that are detected. The fit
parameters are presented in Table 2. The main isotopologues of
CH3OH and HNCO are optically thick and their column densi-
ties are therefore determined from minor isotopologues. This is
done by multiplying with the local interstellar 12C/13 C ratio of
52.5±15.4 (Yan et al. 2019) and 16O/18O ratio of 560 (Wilson
1999).
3.2. IRAS 16293B
Because the chemical inventory of IRAS 16293B is well charac-
terised with results from the PILS survey, this data set is used
to search for HOCH2CN and related species to make an un-
biased chemical comparison with SMM1-a. While HOCH2CN
was identified towards IRAS 16293B by Zeng et al. (2019), this
detection cannot be confirmed in the PILS spectrum at the full-
beam oset position (the position commonly used for the molec-
ular analysis of IRAS 16293B with PILS data, see Fig.3). Fig-
ure 6shows the HOCH2CN a-type transitions (J0
0,J0J00
0,J00 ) at
this position covered by the PILS spectral range with a syn-
thetic glycolonitrile spectrum at NT=1.0×1015 cm2and Tex
=150 and 300 K. A synthetic spectrum of the previously iden-
tified molecules towards IRAS 16293B at the full-beam oset
Article number, page 5 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Fig. (4) Identified lines of HOCH2CN towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(7.4±0.9)×1014 cm2,Tex =260±45 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
position is added in green. Note that this synthetic spectrum
only includes molecules listed in previous publications and does
not include HOCH2CN. The molecules and parameters used in
this fit are listed in Table 5. When a rotational spectrum is ex-
perimentally measured, a-type transitions are usually the first
and most accurately determined transitions. The assignment of
these a-type transitions, therefore, is key to claim an unambigu-
ous identification. However, of the seven a-type transitions cov-
ered by the PILS survey, only the 390,39,1– 380,38,1transition
at 344625 MHz is possibly detected, although this feature has
a contribution of a HONO and CH2DOH transition (Jørgensen
et al. 2016,2018;Coutens et al. 2019), which also can fully re-
produce this line. Appendix Cpresents all the HOCH2CN tran-
sitions at the full-beam oset position that are largely unblended
and have Aij 1×103s1. At the full-beam oset position, a
large number (40) of glycolonitrile transitions that are present
in the synthetic spectrum are not seen in the observed spectrum.
We note that due to the line-richness of the source the baseline
subtraction is challenging and in some cases, it can be oversub-
tracted. This can explain why certain HOCH2CN lines are not
clearly observed, as the baseline at these positions dips. Further-
more, some transitions seem to be better reproduced with low
excitation temperatures, whereas others require warmer temper-
atures. This could indicate that two gas components are traced,
as also shown by Zeng et al. (2019), but at this position these
components are hard to distinguish. Therefore, HOCH2CN can
only tentatively be identified towards the full-beam oset posi-
Article number, page 6 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (5) Identified lines of CH3NCO, ν=0 towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(6.4±1.9)×1014 cm2,Tex =240±60 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
tion, with a column density of 1.0×1015 cm2at Tex =150 and
300 K.
At the half-beam oset position (see Fig. 3), which is closer
to the continuum peak of IRAS 16293B, HOCH2CN can be iden-
tified. At this position, at least four a-type transitions are clearly
detected, while the three other lines suer from line blending or
absorption features, see Fig. 7. These lines can approximately
be fitted with synthetic spectra of NT=3.0×1015 cm2for Tex =
150 and 300 K. In appendix Cthe remaining HOCH2CN transi-
tions at the half-beam oset position are shown, with the same
selection criteria for the full-beam oset position.
In Fig. 3the moment 0 map of the HOCH2CN 364,32
354,31 transition towards IRAS 16293B is shown. This map
shows that HOCH2CN emission towards IRAS 16293B is com-
pact. This explains the non-detection of glycolonitrile towards
the full-beam oset position, as this position misses most of
the HOCH2CN emission. At the same time, this map demon-
strates why Zeng et al. (2019) could detect HOCH2CN towards
IRAS 16293B, since these authors use a larger observational
beam of 100
.6 beam (with an assumed source size of 000
.5), which
covers the entire emitting area.
Because the chemical inventory at the full-beam oset posi-
tion of the PILS data is best characterised, the tentative detec-
tion of HOCH2CN towards this position, with a column density
of 1.0×1015 cm2), is used for further analysis in this paper.
However, the identification of HOCH2CN towards the half-beam
oset position in combination with the moment 0 map support
the detection of HOCH2CN towards IRAS 16923B by Zeng et al.
(2019).
The related species CH2CNH, CH(O)CN, and NH2CH2CN
are also searched for in the PILS dataset, but clear and unblended
lines are not identified. For these species upper limit column den-
sities are determined at the full-beam oset position at excitation
temperatures of 100 and 300 K. The upper limit column densities
and abundances of all species are presented in Table 3.
4. Discussion
In this work, several molecules are detected towards SMM1-a
and analysed. Most notable are the detection of the C2H3NO iso-
mers CH3NCO (methyl isocyanate) and HOCH2CN (glycoloni-
trile). For HOCH2CN, this is only its second interstellar detec-
Article number, page 7 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Fig. (6) Overview of a-type (J0
0,J0J00
0,J00 ) HOCH2CN transitions covered by the PILS spectrum towards the full-beam oset
position of IRAS 16293B, illustrating the non-detection of these lines in this spectrum. The observed spectrum is plotted in black and
synthetic spectra for a column density of 1.0×1015 cm2and excitation temperatures of 150 (blue) and 300 K (red) are overplotted.
The synthetic spectrum of the entire molecular inventory determined with PILS data towards this position is plotted in green. The
quantum numbers of the transition are indicated at the top of each panel and the upper state energy is given in the top left of each
panel.
Fig. (7) Overview of a-type (J0
0,J0J00
0,J00 ) HOCH2CN transitions covered by the PILS spectrum towards the half-beam oset
position of IRAS 16293B, illustrating the detection of a number of these lines. The observed spectrum is plotted in black and
synthetic spectra for a column density of 3.0×1015 cm2and excitation temperatures of 150 (blue) and 300 K (red) are overplotted.
The quantum numbers of the transition are indicated at the top of each panel and the upper state energy is given in the top left of
each panel.
tion. CH3NCO has been detected in multiple interstellar sources,
but this is the first detection towards SMM1 and therefore also
the first detection towards an intermediate-mass source. These
new detections serve as additional evidence for a large and di-
verse reservoir of prebiotic molecules in star- and planet-forming
regions, which can contribute to the emergence of biomolecules
on planetary bodies. Of the C2H3NO isomers, CH3NCO is en-
ergetically the most favorable, followed by HOCH2CN, which
has a higher relative energy of 12.1 – 18.6 kcal mol1(0.5 – 0.8
eV molecule1or 5800 – 9300 K molecule1), depending on the
level of theory used (Fourré et al. 2020). In a thermodynamic
equilibrium, lower energy or more stable products are favored
and in such a scenario, CH3NCO is expected to be more abun-
dant than HOCH2CN by a factor of at least 1×109(assuming a
Article number, page 8 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
temperature of 300 K). The fact that CH3NCO and HOCH2CN
are found to be equally abundant is, therefore, evidence that the
formation of these molecules is rather driven by kinetics.
To better understand the interstellar chemistry of HOCH2CN
and CH3NCO, their abundances and those of several other
species are compared for SMM1-a, IRAS 16293B, and several
other sources for which at least some of these species are de-
tected. For this comparison, it is important to emphasise obser-
vational dierences. For example, SMM1 (D 436 pc) is lo-
cated further away than IRAS 16293 (D 141 pc, Dzib et al.
2018) and the beam size used in this work (100
.2) is larger than
that of the PILS survey (000
.5). Therefore, the chemical inven-
tory of SMM1-a and the involved chemical processes are inves-
tigated on a much larger spatial scale of roughly 500 au, com-
pared to about 70 au for IRAS 16293B. The SMM1-a obser-
vations can cover a larger range of physical environments (e.g.
also the envelope or outflow), and potentially a larger tempera-
ture gradient. At the same time, the higher luminosity of SMM1
results in a larger area where hot core conditions are present, thus
compensating for the lower spatial resolution. Furthermore, the
SMM1-a spectrum is extracted towards the continuum peak, but
for IRAS 16293B a full-beam oset position from the contin-
uum peak is used. Therefore, when comparing molecular ratios
between the two sources, they may not only be aected by dier-
ent physical conditions but also due to the way the sources were
observed. Observational parameters of SMM1-a, IRAS 16293B,
and other sources used for comparisons are listed in Table D.1.
HOCH2CN is thus far only detected in two sources, SMM1-
a and IRAS 16293B (Zeng et al. 2019), and therefore a chem-
ical comparison is limited to these two objects. To use molec-
ular ratios that are unbiased by observational parameters, only
data of the chemical inventory of IRAS 16293B obtained with
PILS survey data at the full-beam oset position is used for the
source comparison. This means that the tentative column den-
sity of HOCH2CN in IRAS 16293B is used. Figure 8shows the
abundances of molecules detected in this work to CH3OH and
HNCO in SMM1-a and IRAS 16293B. For IRAS 16293B, the
analysis performed in this work is combined with results from
Jørgensen et al. (2016); Coutens et al. (2016); Ligterink et al.
(2017); Coutens et al. (2018); Persson et al. (2018); Calcutt et al.
(2018); Jørgensen et al. (2018). For both the [X]/[CH3OH] and
[X]/[HNCO] ratios, all the oxygen-bearing molecules, CH3CN,
and NH2CN are found to be more abundant in IRAS 16293B
than in SMM1-a. For [X]/[CH3OH] its ratios are generally a fac-
tor of a few lower in SMM1-a, while for the [X]/[HNCO] ratios
the dierence is usually more than a factor of ten.
4.1. SMM1-a: A HOCH2CN-rich source
While general trends are found in the [X]/[CH3OH] and
[X]/[HNCO] ratios displayed in Fig. 8, three molecules de-
viate from the general trend. The abundance of CH3NCO
is found to be approximately equal in both sources, while
HOCH2CN and HNCO are more abundant in SMM1-a com-
pared to IRAS 16293B. In particular, for HOCH2CN a large dif-
ference is seen in its ratios to CH3OH, which can be more than
an order of magnitude dierent between the two sources.
To gain further insight in how the chemical compositions
of SMM1-a and IRAS 16293B dier, the statistical distance of
molecular ratios are plotted in Fig. 9. The statistical distance in-
dicates how significant the dierence in a molecular ratio be-
tween SMM1-a and IRAS 16293B is (see Manigand et al. 2020,
and Appendix E). Greater values indicate a greater dierence
in the molecular ratios between the two sources. A positive
value indicates that the ratio of SMM1-a is greater than that of
IRAS 16293B and vice versa. Fig. 9highlights that HOCH2CN
and HNCO are more abundant in SMM1-a (σranging from 3 –
6) and CH3NCO is moderately more abundant in SMM1-a (σ
2).
The statistical distance results have several implications.
Since the abundances of both HOCH2CN and HNCO are en-
hanced in SMM1-a, this may indicate a relationship between
these two species. For CH3NCO a much less significant en-
hancement is seen, which can imply that both C2H3NO isomers
form via dierent chemical reactions or under dierent physical
conditions. However, it is important to stress that an abundance
correlation does not always imply a formational link between
species (Belloche et al. 2020).
The statistical distances of ratios involving CH3CN, NH2CN,
a-(CH2OH), and CH3CH2OH show that there is almost no vari-
ation in these molecules between SMM1-a and IRAS 16293B.
This is interesting because some of these molecules can form in
reactions involving radicals from which HOCH2CN also can be
formed, such as CN, CH2OH, and CH2CN. This hints that either
HOCH2CN is not formed from these radicals, is not linked to
the reaction networks that form the four aforementioned species,
or HOCH2CN forms from the same radicals, but under dierent
physical conditions.
Finally, it is interesting to note that ratios of D2CO and
CH3OCHO to NH2CN, CH3CH2OH, and a-(CH2OH)2seem to
be a bit more abundant in SMM1-a than in IRAS 16293B. This
is particularly significant for the case of D2CO since H2CO is
involved in the Strecker-like formation of HOCH2CN, see re-
action 2. A higher abundance of H2CO may indicate that the
Strecker-like reaction can more eciently take place. However,
care needs to be taken with this interpretation, since the D2CO
spectral lines in the SMM1-a spectrum are blended and thus
there is a large uncertainty on its column density. At the same
time, the D/H ratio of H2CO is not known in SMM1-a, which
introduces another source of uncertainty. To investigate if there
is a correlation between HOCH2CN and H2CO, both species
should be identified towards more sources and H2CO should be
observed through its minor 13C and 18O isotopes instead of the
deuterated species. For now, however, the Strecker-like synthe-
sis of HOCH2CN in the ISM can neither be confirmed nor ruled
out.
4.2. Interstellar formation of CH3NCO
Since its first detection in the ISM, the formation of CH3NCO
has been hypothesised to be linked to that of HNCO, see
Eq. 1. To investigate the interstellar chemical relationship be-
tween these two species, their gas-phase ratios towards inter-
stellar sources are plotted in Fig. 10. The result of the SMM1-
a analysis is complemented by data from the low-mass proto-
star IRAS 16239B (Ligterink et al. 2017), the quiescent giant
molecular cloud G+0.693 (Zeng et al. 2018), the high-mass star-
forming region Orion KL (Cernicharo et al. 2016), the high-mass
hot molecular core G10.47+0.03 (Gorai et al. 2020), and the
galactic center source Sagittarius B2(N) (Sgr B2(N), Belloche
et al. 2013;Cernicharo et al. 2016;Belloche et al. 2017). The
majority of these sources is categorised as hot cores or corinos.
In these sources, thermal desorption of molecules from ice man-
tles plays an important role to chemically enrich the gas sur-
rounding the protostar, in particular around the desorption tem-
perature of water ice (100 K). The exception is the source
G+0.693, which is a molecular cloud. Molecules observed in
the gas of this cloud are assumed to be the result of gas-phase
Article number, page 9 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
NH2CNg
D2C O f
CH3CNe
a - ( C H 2O H ) 2d
CH3CH2O H d
CH3O C H O d
HNCOc
CH3NCOb
HOCH2CNa
1 E - 3 0 .0 1 0 . 1 1 1 0
~
( % )
[X ] / [C H 3O H ] ( % )
S M M 1
I R A S 1 6 2 9 3 B ~
NH2CNg
D2C O f
CH3CNe
a - ( C H 2O H ) 2d
CH3CH2O H d
CH3O C H O d
CH3O H d
CH3NCOb
HOCH2CNa
0.1 1 10 100 1000 10000
S M M 1
I R A S 1 6 2 9 3 B
[ X ] / [ H N C O ] ( % )
( % )
Fig. (8) Ratios of [X]/[CH3OH] towards SMM1-a (red) and IRAS 16293B (blue) in decreasing order of SMM1-a abundance.
The “” symbol indicates that these HOCH2CN ratios have been determined with the column density of the tentative HOCH2CN
detection towards the IRAS 16293B full-beam oset position. For IRAS 16293B, column densities derived towards the full-beam
oset position from the following PILS publications are used: aThis work, bLigterink et al. (2017), cCoutens et al. (2016), dJørgensen
et al. (2016,2018), eCalcutt et al. (2018), fPersson et al. (2018), gCoutens et al. (2018).
Fig. (9) Statistical distance between molecular ratios in SMM1
and IRAS 16293B, given in σ. Larger σvalues indicate a larger
dierence between the two sources for a given ratio. Positive
values indicate that a ratio is higher in SMM1-a, while negative
values indicate that a ratio is lower in SMM1-a. In particular, all
ratios of HOCH2CN are found to be higher in SMM1-a than in
IRAS 16239B.
formation reactions or non-thermal desorption from ice mantles
of dust grains.
The ratios of CH3NCO and HNCO are generally similar at
[CH3NCO] /[HNCO] =10% and vary only by a factor of a
few, in particular when the lowest and highest ratio are omitted.
The lowest ratio, found towards G+0.693, arises in a source with
a very dierent physical structure and can therefore not directly
be compared with the hot core and corino sources. The highest
ratio is found towards G10.47+0.03, but the analysis of HNCO
in this source is likely performed on optically thick HNCO lines
and the analysis seems to underestimate the HNCO column den-
sity in this source (see Fig. 4 in Gorai et al. 2020). Removing
these results from the analysis, a correlation between CH3NCO
1 E 1 6 1 E 1 7 1 E 1 8
1 E 1 4
1 E 1 5
1 E 1 6
1 E 1 7
1 E 1 8
G 10.47+ 0.03
S g r B 2 ( N ) c 1
S g r B 2 ( N ) c 2
I R A S 1 6 2 9 3 B
S g r B 2 ( N 2 )
O r i o n K L A
O r i o n K L B
S M M 1 - a
G + 0 . 6 9 3
10%
100%
N(CH3N C O ) [ c m - 2 ]
N ( H N C O ) [ c m - 2 ]
1 %
Fig. (10) Ratios of [CH3NCO]/[HNCO] ratios towards
SMM1-a and various other sources. Column densities from the
following publications are used: Belloche et al. (2013), Cer-
nicharo et al. (2016), Ligterink et al. (2017), Belloche et al.
(2017), Zeng et al. (2018), Gorai et al. (2020), and this work.
and HNCO is found, which may indicate a chemical link be-
tween CH3NCO and HNCO. Furthermore, this correlation spans
a variety of sources of dierent masses and over four order of
magnitude in luminosities. This results in dierent physical con-
ditions, such as gas and dust temperature and radiation fields
for each source. Therefore, the lack of source-to-source varia-
tion in [CH3NCO]/[HNCO] ratio hints that the abundances of
these species are set at an early stage of star formation. Forma-
tion of CH3NCO via reaction 1in the ice mantles of dust grains
during the dark cloud stage or while the ice mantles are warmed
up in the hot core/corino stage are a plausible scenario.
Article number, page 10 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
4.3. On the formation of -CN and -NCO molecules
Figure 9shows that HOCH2CN and HNCO abundances are en-
hanced in SMM1-a compared to IRAS 16293B. At the same
time, CH3NCO is only marginally enhanced and CH3CN and
NH2CN abundances show little dierence between the two
sources. Why the abundances of HOCH2CN and HNCO are en-
hanced in SMM1-a and those of CH3NCO, CH3CN, and NH2CN
are not is not straightforward to explain. However, it is likely that
the various -CN and -NCO molecules form in dierent chemical
reactions and physical conditions.
As this work shows, CH3NCO probably forms at an early
stage of star-formation, as do CH3CN and NH2CN. All three
species are suggested to form in radical-radical addition re-
actions in ice mantles. These reactions take place during the
dark cloud stage in cold (10 K) ice mantles and significantly
speed up when the ice mantle temperature increases to 30 K
and radicals become mobile (Garrod et al. 2008;Coutens et al.
2018). Some reactions can compete for the same radical, such as
CH3CN and NH2CN, which both compete for the -CN radical.
The fact that HOCH2CN and HNCO are enhanced in abun-
dance in SMM1-a, can indicate a link between these species.
Both molecules can be formed from HCN in ice mantles of inter-
stellar dust grains. HOCH2CN can be formed in the Strecker-like
reaction when HCN is converted to CN. HNCO and the related
anion OCNare formed when HCN:H2O mixtures are processed
with energetic UV photons or protons (Gerakines et al. 2004).
These reactions are aided by high grain temperatures (30 K,
but below the water sublimation temperature) for a prolonged
time and high fluxes of photons and energetic particles. If these
conditions are met in SMM1, they can explain the higher abun-
dances of HOCH2CN and HNCO compared to IRAS 16239B
and present a formational link between some -CN and -NCO
molecules. However, only circumstantial evidence can be pre-
sented for such conditions in SMM1, which is based on the
fact that SMM1 hosts multiple protostellar sources and outflows,
which can warm and irradiate the cloud (e.g. Choi 2009;Dion-
atos et al. 2014;Hull et al. 2017;Tychoniec et al. 2019).
To gain further insight into the reactions that form -CN and -
NCO molecules, unbiased observations of these species towards
a multitude of sources spanning dierent physical conditions are
needed. Not only the isomers HOCH2CN and CH3NCO should
be targeted for these observations, but also species like CH3CN,
HNCO, NH2CN, C2H3CN, and C2H5CN. Laboratory, theoret-
ical and modeling eorts should be focused on understanding
the formation of these species. In particular, the formation of
HOCH2CN via pathways other than the thermal Strecker-like
synthesis needs to be studied and a better understanding of the
formation of CH3CN is required.
5. Conclusions
This publication presents the simultaneous detection of the
C2H3NO isomers methyl isocyanate (CH3NCO) and glycoloni-
trile (HOCH2CN). Both species are identified towards the
intermediate-mass Class 0 protostar Serpens SMM1-a. This
is only the second interstellar detection of glycolonitrile,
while for methyl isocyanate it is the first detection towards
an intermediate-mass protostar. Additionally, CH3OH, HNCO,
CH3OCHO, CH3CH2OH, a-(CH2OH)2, D2CO, and NH2CN are
detected. CH2CNH, CH(O)CN, and NH2CH2CN, molecules that
are related to HOCH2CN, are searched for but not identified.
Data from the PILS survey towards IRAS 16293B are analysed
in search for HOCH2CN and this molecule is identified in a spec-
trum extracted at a half-beam oset position from the continuum
peak of IRAS 16239B. The molecules CH2CNH, CH(O)CN, and
NH2CH2CN are not identified towards this source.
The detection of CH3NCO and HOCH2CN towards SMM1-
a is additional evidence of a large interstellar reservoir of pre-
biotic molecules. Delivery of these molecules to planetary sur-
faces may contribute to the formation of biomolecules on these
objects. The column densities and abundances of CH3NCO (NT
=6.4×1014 cm2and [CH3NCO]/[CH3OH] =5.3×104) and
HOCH2CN (NT=7.4×1014 cm2and [HOCH2CN]/[CH3OH]
=6.2×104) are found to be equal within their error bars. Since
HOCH2CN is the least energetically favorable of the two iso-
mers, thermodynamics predicts that CH3NCO should be more
abundant. The equal ratio between both molecules is therefore
evidence that the formation of these molecules is driven by ki-
netics.
The comparison of molecular ratios between SMM1-a and
IRAS 16293B show that HOCH2CN and HNCO are signifi-
cantly more abundant in the former source. The molecular ratios
of HOCH2CN hint that the formation of this molecule does not
heavily depend on solid-state radical-radical addition reactions,
such as HOCH2+CN and HO +CH2CN. Formation via the
thermal Strecker-like reaction [X+CN]+H2CO in ice mantles
cannot be confirmed nor ruled out based on the current data but
may be a prominent formation pathway.
To investigate the possibility that CH3NCO formation is re-
lated to HNCO, the ratios of these molecules in SMM1-a and
other sources are analysed. These ratios are found to be uni-
form throughout all sources at [CH3NCO]/[HNCO] =10 %.
This indicates that there is a chemical link between both species,
but also that its ratios is already set at an early stage of star-
formation. Presumably CH3NCO forms via the radical-radical
reaction CH3+NCO in ice mantles during the dark cloud stage.
It is dicult to establish a chemical link between -CN and
-NCO molecules. Some may be related, such as HOCH2CN and
HNCO, which can both form from HCN at elevated (30 K)
grain temperatures and in relatively high radiation fields. Contin-
ued observational, laboratory, and theoretical studies of -CN and
-NCO molecules are required to gain further insight into their
formation and links in their chemistry.
Acknowledgements. We thank E.G. Bøgelund, S.F. Wampfler, M.N. Droz-
dovskaya, B. Kulterer, and B.A. McGuire for helpful discussions on the ob-
servations, spectroscopy, and the chemistry of the C2H3NO isomers. The au-
thors acknowledge assistance from Allegro, the European ALMA Regional
Center node in the Netherlands. We thank the anonymous referee for their
thorough review of this manuscript and helpful comments. This paper makes
use of the following ALMA data: ADS/JAO.ALMA#2018.1.00836.S and
ADS/JAO.ALMA#2013.1.00278.S. ALMA is a partnership of ESO (represent-
ing its member states), NSF (USA) and NINS (Japan), together with NRC
(Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in coop-
eration with the Republic of Chile. The Joint ALMA Observatory is operated by
ESO, AUI/NRAO and NAOJ. NFWL is supported by the Swiss National Science
Foundation (SNSF) Ambizione grant 193453. JKJ is supported by the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme through ERC Consolidator Grant “S4F” (grant agreement
No 646908). AC acknowledges financial support from the Agence Nationale de
la Recherche (grant ANR-19-ERC7-0001-01).
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Article number, page 12 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Appendix A: Spectroscopic data
In this paper, the CDMS and JPL spectroscopic databases are the
primary sources of molecular line lists. In the following table, an
overview of the analysed molecules, their identifier and catalog,
and the most important publications in literature on which these
entries are based is given.
Appendix B: Supporting information for the
SMM1-a analysis
Appendix C: Analysis of PILS data
Appendix D: Source parameters
Appendix E: Statistical distance
The statistical distance of molecular ratios between SMM1-a and
IRAS 16293B is calculated according to the following equation:
SX/Y=NX
NYSMM1aNX
NYIRAS 16293B
qσ2
SMM1a+σ2
IRAS 16293B
,(E.1)
where Nxand Nyare the column densities of two dierent
molecules and σis the uncertainty on the column density ratio
NX
NY. The value of SX/Yis given in σand indicates the signif-
icance of the dierence, with greater values implying that there
is a more significant dierence between the two sources. In this
equation, positive values indicate that NX
NYis more abundant in
SMM1-a than in IRAS 16293B, while negative values indicate
the opposite.
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Table (A.1)
Molecule ID catalog entry date reference
D2CO 32502 CDMS Jan 2016 Bocquet et al. (1999)
Zakharenko et al. (2015)
12CH3OH 32504 CDMS May 2016 Xu et al. (2008)
CH18
3OH 34504 CDMS Sep 2020 Fisher et al. (2007)
CH3CN, ν8=1 41509 CDMS Nov 2016 Müller et al. (2015)
Koivusaari et al. (1992)
NH2CN 42003 JPL Jan 1991 Read et al. (1986)
HN12CO 43511 CDMS May 2009 Kukolich et al. (1971)
Hocking et al. (1975)
Niedenhoet al. (1995)
Lapinov et al. (2007)
HN13CO 44008 JPL Jul 1987 Hocking et al. (1975)
CH3CH2OH 46524 CDMS Nov 2016 Pearson et al. (2008)
Müller et al. (2016)
CH3NCO, ν=0 57505 CDMS Mar 2016 Cernicharo et al. (2016)
CH3NCO, ν=1 57506 CDMS Mar 2016 Cernicharo et al. (2016)
HOCH2CN 57512 CDMS Mar 2017 Margulès et al. (2017)
CH3OCHO 60003 JPL Apr 2009 Ilyushin et al. (2009)
a-(CH2OH)262503 CDMS Sep 2003 Christen et al. (1995)
Christen & Müller (2003)
Fig. (B.1) Identified lines of CH3CN ν8=1 towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(1.3±0.3)×1015 cm2,Tex =190±25 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
Table (D.1) Physical parameters of interstellar sources used for abundance comparison.
Source Telescope Distance Luminosity Beam size physical size Reference
(pc) L(00 ×00) au
IRAS 16293–2422B ALMA 141 3a0.5×0.5 70 Jørgensen et al. (2016)
Serpens SMM1-a ALMA 436 100b1.3×1.0 500 this work
Orion KL ALMA 414 1×1051.8×1.8 750 Cernicharo et al. (2016)
Sgr B2(N2) ALMA 8300 4.7×1061.6×1.2 – 2.9×1.5 1.3×104Belloche et al. (2017)
G10.47+0.03 ALMA 8550 5×1052.0×1.4 – 2.4×1.6 1.6×104Gorai et al. (2020)
Sgr B2(N) IRAM 30m 8300 4.7×1063.2×2.8 – 12.2×4.4 3.0×104Cernicharo et al. (2016)
G+0.693 IRAM 30m & GBT 8300c– 9×9 – 55×55 7.5×104Zeng et al. (2019)
Notes. aLuminosity determined from a modeling investigation by Jacobsen et al. (2018). bLuminosity for the entire SMM1 source. cThe distance
to G+0.693 is assumed to be the same as Sgr B2.
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N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (B.2) Identified lines of NH2CN towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spec-
trum overplotted in blue (NT=(5.1±1.3)×1013 cm2,Tex =190±40 K). The transition is indicated at the top of each panel and the
upper state energy is given in the top left of each panel.
Fig. (B.3) Identified lines of a-(CH2OH)2towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(1.7±0.5)×1015 cm2,Tex =195±70 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
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Fig. (B.4) Identified lines of CH3CH2OH towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(4.1±0.9)×1015 cm2,Tex =210±25 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
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N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (B.5) Identified lines of CH3OCHO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(7.4±0.7)×1015 cm2,Tex =215±20 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
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Fig. (B.6) Identified lines of D2CO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum
overplotted in blue (NT=(5.4±0.5)×1014 cm2,Tex =[200] K). The transition is indicated at the top of each panel and the upper
state energy is given in the top left of each panel.
Fig. (B.7) Identified lines of HN12CO towards SMM1. The observed spectrum is plotted in black and the line position is indicated
by the red dotted line. Because these lines are optically thick, not synthetic fit is given. The transition is indicated at the top of each
panel and the upper state energy is given in the top left of each panel.
Fig. (B.8) Identified lines of HN13CO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(1.9±0.3)×1014 cm2,Tex =190±30 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
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N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (B.9) Identified lines of 12CH3OH towards SMM1. The observed spectrum is plotted in black and the line position is indicated
by the red dotted line. Because these lines are optically thick, not synthetic fit is given. The transition is indicated at the top of each
panel and the upper state energy is given in the top left of each panel.
Fig. (B.10) Identified lines of CH18
3OH towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic
spectrum overplotted in blue (NT=(2.0±0.7)×1015 cm2,Tex =250±60 K). The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel.
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Fig. (C.1) Spectral lines of HOCH2CN in the PILS spectrum towards IRAS 16293B at the full-beam oset position. The observed
spectrum is plotted in black, with synthetic spectra overplotted (NT=[1.0×1015] cm2,Tex =150, blue, and 300 K, red). The
synthetic spectrum of the entire molecular inventory determined with PILS data towards this position is plotted in green. All
covered transitions with Aij 1.0×103s1that are not blended are shown. The transition is indicated at the top of each panel and
the upper state energy is given in the top left of each panel. HOCH2CN is not detected in the full-beam oset position spectrum
towards IRAS 16293B.
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N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (C.2) Same as Fig. C.1
Article number, page 21 of 32
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Fig. (C.3) Same as Fig. C.1
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N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (C.4) Spectral lines of HOCH2CN in the PILS spectrum towards IRAS 16293B at the half-beam oset position. The observed
spectrum is plotted in black, with synthetic spectra overplotted (NT=[3.0×1015] cm2,Tex =150, blue, and 300 K, red). All covered
transitions with Aij 1.0×103s1that are not blended are shown. The transition is indicated at the top of each panel and the
upper state energy is given in the top left of each panel. HOCH2CN is detected in the half-beam oset position spectrum towards
IRAS 16293B.
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Fig. (C.5) Same as Fig. C.4
Article number, page 24 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Fig. (C.6) Same as Fig. C.4
Article number, page 25 of 32
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Table (B.1) Spectral information of molecules detected towards SMM1-a
Molecule Database entry Transition Frequency Eup Aij
J,Ka,Kc,(F) (MHz) (K) s1
D2CO 32502 4 2 3 - 3 2 2 233 650.441 (0.0500) 49.63 2.69×104
CDMS 4 3 2 - 3 3 1 234 293.361 (0.0500) 76.62 1.58×104
4 3 1 - 3 3 0 234 331.062 (0.0500) 76.63 1.58×104
12CH3OH 32504 15 6 9 3 - 16 5 11 3 217 642.677 (0.0220) 746 1.89×105
CDMS 15 6 10 3 - 16 5 12 3 217 642.678 (0.0220) 746 1.89×105
4 2 3 1 - 3 1 2 1 218 440.063 (0.0130) 45 5.69×105
32 2 30 2 - 32 1 31 1 234 523.365 (0.1070) 1256 8.95×105
4 2 3 0 - 5 1 4 0 234 683.370 (0.0120) 61 1.87×105
5 4 2 2 - 6 3 3 2 234 698.519 (0.0150) 123 6.34×106
CH18
3OH 34504 5 0 5 0 - 4 0 4 0 231 758.446 (0.0300) 33 5.33×105
CDMS 5 3 3 0 - 4 3 2 0 231 796.218 (0.0300) 83 3.41×105
5 3 2 0 - 4 3 1 0 231 796.521 (0.0300) 83 3.41×105
5 3 2 2 - 4 3 1 2 231 801.304 (0.0300) 81 3.42×105
5 2 4 0 - 4 2 3 0 231 801.466 (0.0300) 71 4.53×105
5 1 4 2 - 4 1 3 2 231 826.744 (0.0300) 54 5.33×105
5 2 3 2 - 4 2 2 2 231 864.501 (0.0300) 56 4.41×105
CH3CN, ν8=1 41509 12 2 2 - 11 -2 2 221 367.450 (0.0011) 649 8.98×104
CDMS 12 -2 2 - 11 2 2 221 367.450 (0.0011) 649 8.98×104
12 4 3 - 11 -4 3 221 380.608 (0.0011) 655 8.21×104
12 -4 3 - 11 4 3 221 380.608 (0.0011) 655 8.21×104
12 1 2 - 11 1 2 221 387.271 (0.0011) 615 9.17×104
12 0 2 - 11 0 2 221 394.085 (0.0011) 594 9.24×104
12 3 3 - 11 3 3 221 403.521 (0.0011) 619 8.66×104
12 1 3 - 11 -1 3 221 625.840 (0.0011) 588 9.20×104
NH2CN 42003 11 1 11 0 - 10 1 10 0 218 461.795 (0.0200) 77 1.08×103
JPL 11 0 11 1 - 10 0 10 1 219 719.651 (0.0200) 135 1.08×103
11 1 10 0 - 10 1 9 0 221 361.160 (0.0200) 78 1.12×103
HN12CO 43511 10 1 10 - 9 1 9 218 985.696 (0.0192) 101 1.48×104
CDMS 10 2 9 - 9 2 8 219 733.850 (0.0300) 228 1.35×104
10 2 8 - 9 2 7 219 737.193 (0.0300) 228 1.35×104
10 0 10 - 9 0 9 219 798.274 (0.0040) 58 1.47×104
28 1 28 - 29 0 29 231 873.255 (0.0064) 470 6.68×105
HN13CO 44008 10 1 10 9 - 9 1 9 9 218 984.716 (0.1053) 101 1.63×106
10 1 10 11 - 9 1 9 10 218 985.697 (0.0192) 101 1.48×104
10 1 10 10 - 9 1 9 9 218 985.705 (0.0191) 101 1.46×104
10 1 10 9 - 9 1 9 8 218 985.706 (0.0191) 101 1.46×104
10 1 10 10 - 9 1 9 10 218 986.596 (0.0191) 101 1.48×106
10 1 10 10 - 9 1 9 10 218 986.596 (0.0191) 101 1.48×106
10 2 9 9 - 9 2 8 9 219 739.762 (0.0962) 231 1.60×106
10 2 9 11 - 9 2 8 10 219 740.451 (0.0274) 231 1.45×104
10 2 9 9 - 9 2 8 8 219 740.456 (0.0274) 231 1.43×104
10 2 9 10 - 9 2 8 9 219 740.471 (0.0274) 231 1.43×104
10 2 9 10 - 9 2 8 10 219 741.095 (0.0885) 231 1.45×106
10 2 8 9 - 9 2 7 9 219 743.054 (0.0966) 231 1.60×106
10 2 8 11 - 9 2 7 10 219 743.742 (0.0288) 231 1.45×104
10 2 8 9 - 9 2 7 8 219 743.747 (0.0288) 231 1.43×104
10 2 8 10 - 9 2 7 9 219 743.762 (0.0288) 231 1.43×104
10 2 8 10 - 9 2 7 10 219 744.386 (0.0890) 231 1.45×106
10 0 10 9 - 9 0 9 9 219 803.645 (0.1070) 58 1.67×106
10 0 10 11 - 9 0 9 10 21 9804.439 (0.0171) 58 1.51×104
10 0 10 10 - 9 0 9 9 219 804.442 (0.0171) 58 1.50×104
10 0 10 9 - 9 0 9 8 219 804.446 (0.0171) 58 1.49×104
10 0 10 10 - 9 0 9 10 219 805.163 (0.0965) 58 1.51×106
CH3CH2OH 46524 35 4 31 2 - 35 3 32 2 218 943.289 (0.0068) 560 7.38×105
CDMS 34 6 28 2 - 34 5 29 2 221 612.904 (0.0080) 548 8.84×105
31 5 27 1 - 31 4 27 0 231 789.850 (0.0120) 506 2.24×105
Article number, page 26 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Table (B.1) Continued.
Molecule Database entry Transition Frequency Eup Aij
J,Ka,Kc,(F) (MHz) (K) s1
22 5 18 2 - 22 4 19 2 231 790.056 (0.0035) 245 8.46×105
13 5 8 2 - 13 4 9 2 233 571.051 (0.0032) 108 7.54×105
14 5 10 2 - 14 4 11 2 233 601.554 (0.0032) 120 7.71×105
12 5 8 2 - 12 4 9 2 234 255.240 (0.0032) 97 7.39×105
11 5 6 2 - 11 4 7 2 234 406.433 (0.0033) 87 7.17×105
10 5 5 2 - 10 4 6 2 234 666.142 (0.0035) 78 6.89×105
10 5 6 2 - 10 4 7 2 234 714.782 (0.0035) 78 6.90×105
23 1 22 2 - 23 0 23 2 234 725.620 (0.0090) 233 3.17×105
9 5 4 2 - 9 4 5 2 234 852.862 (0.0038) 69 6.53×105
36 4 32 2 - 36 3 33 2 234 855.010 (0.0073) 591 8.70×105
9 5 5 2 - 9 4 6 2 234 873.873 (0.0038) 69 6.54×105
14 2 12 1 - 13 3 10 0 234 882.537 (0.0105) 155 3.64×105
14 0 14 1 - 13 1 12 0 235 158.494 (0.0058) 146 1.12×104
CH3NCO, ν=0 57505 25 -1 0 2 - 24 -1 0 2 217 595.174 (0.0500) 194 4.84×104
CDMS 25 0 0 2 - 24 0 0 2 217 595.174 (0.0500) 188 4.85×104
25 2 0 1 - 24 2 0 1 217 652.088 (0.0500) 171 4.82×104
24 3 0 1 - 23 3 0 1 217 701.086 (0.0500) 191 4.40×104
25 1 0 -3 - 24 1 0 -3 218 002.461 (0.0500) 258 4.93×104
25 0 0 -3 - 24 0 0 -3 218 014.630 (0.0500) 251 4.84×104
25 1 0 3 - 24 1 0 3 218 069.900 (0.0500) 257 4.93×104
25 1 24 0 - 24 1 23 0 218 541.803 (0.0500) 142 4.94×104
27 -1 0 1 - 26 -1 0 1 231 793.783 (0.0500) 175 6.02×104
27 2 26 0 - 26 2 25 0 234 088.125 (0.0500) 181 6.05×104
27 0 0 2 - 26 0 0 2 234 932.492 (0.0500) 210 6.11×104
27 -3 0 2 - 26 -3 0 2 235 663.096 (0.0500) 264 6.06×104
27 2 0 3 - 26 2 0 3 235 801.163 (0.0500) 296 6.12×104
27 2 0 -3 - 26 2 0 -3 235 803.211 (0.0500) 297 6.12×104
HOCH2CN 57512 24 2 23 1 - 23 2 22 1 218 994.156 (0.0009) 143 3.23×104
CDMS 24 8 17 1 - 23 8 16 1 221 334.546 (0.0009) 227 2.98×104
24 8 16 1 - 23 8 15 1 221 334.546 (0.0009) 227 2.98×104
24 9 15 1 - 23 9 14 1 221 344.331 (0.0010) 251 2.88×104
24 9 16 1 - 23 9 15 1 221 344.331 (0.0010) 251 2.88×104
24 10 14 1 - 23 10 13 1 221 372.125 (0.0010) 277 2.77×104
24 10 15 1 - 23 10 14 1 221 372.125 (0.0010) 277 2.77×104
24 6 19 1 - 23 6 18 1 221 406.933 (0.0009) 188 3.15×104
24 6 18 1 - 23 6 17 1 221 407.170 (0.0009) 188 3.15×104
24 11 13 1 - 23 11 12 1 221 413.638 (0.0010) 306 2.65×104
24 11 14 1 - 23 11 13 1 221 413.638 (0.0010) 306 2.65×104
24 12 12 1 - 23 12 11 1 221 466.304 (0.0011) 338 2.52×104
24 12 13 1 - 23 12 12 1 221 466.304 (0.0011) 338 2.52×104
24 3 22 1 - 23 3 21 1 221 466.604 (0.0009) 151 3.32×104
24 8 17 0 - 23 8 16 0 221 480.199 (0.0009) 222 2.95×104
24 8 16 0 - 23 8 15 0 221 480.199 (0.0009) 222 2.95×104
24 9 15 0 - 23 9 14 0 221 488.785 (0.0010) 245 2.86×104
24 9 16 0 - 23 9 15 0 221 488.785 (0.0010) 245 2.86×104
24 7 18 0 - 23 7 17 0 221 497.997 (0.0009) 201 3.04×104
24 7 17 0 - 23 7 16 0 221 498.002 (0.0009) 201 3.04×104
24 10 14 0 - 23 10 13 0 221 515.800 (0.0010) 272 2.75×104
24 10 15 0 - 23 10 14 0 221 515.800 (0.0010) 272 2.75×104
24 13 11 1 - 23 13 10 1 221 528.501 (0.0011) 372 2.38×104
24 13 12 1 - 23 13 11 1 221 528.501 (0.0011) 372 2.38×104
24 5 20 1 - 23 5 19 1 221 533.058 (0.0009) 173 3.22×104
24 5 19 1 - 23 5 18 1 221 542.097 (0.0009) 173 3.22×104
24 11 13 0 - 23 11 12 0 221 556.802 (0.0010) 301 2.63×104
24 11 14 0 - 23 11 13 0 221 556.802 (0.0010) 301 2.63×104
24 6 19 0 - 23 6 18 0 221 557.660 (0.0009) 183 3.12×104
24 6 18 0 - 23 6 17 0 221 557.911 (0.0009) 183 3.12×104
Article number, page 27 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Table (B.1) Continued.
Molecule Database entry Transition Frequency Eup Aij
J,Ka,Kc,(F) (MHz) (K) s1
24 12 12 0 - 23 12 11 0 221 609.140 (0.0010) 332 2.50×104
24 12 13 0 - 23 12 12 0 221 609.140 (0.0010) 332 2.50×104
25 2 23 1 - 24 2 22 1 234 584.932 (0.0300) 157 4.36×104
25 2 23 0 - 24 2 22 0 235 112.110 (0.0300) 152 3.94×104
CH3OCHO 60003 18 10 9 3 - 17 10 8 3 219 822.126 (0.1000) 355 1.11×104
JPL 18 10 8 3 - 17 10 7 3 219 822.126 (0.1000) 355 1.11×104
18 11 7 0 - 17 11 6 0 221 433.019 (0.1000) 181 1.03×104
18 11 8 0 - 17 11 7 0 221 433.019 (0.1000) 181 1.03×104
18 10 8 2 - 17 10 7 2 221 649.411 (0.1000) 167 1.14×104
19 4 16 4 - 18 4 15 4 231 896.060 (0.1000) 310 1.79×104
19 8 12 4 - 18 8 11 4 233 627.478 (0.1000) 341 1.59×104
19 10 10 0 - 18 10 9 0 234 124.883 (0.1000) 179 1.40×104
19 10 9 0 - 18 10 8 0 234 124.883 (0.1000) 179 1.40×104
19 10 10 1 - 18 10 9 1 234 134.600 (0.0500) 178 1.40×104
19 6 14 3 - 18 6 13 3 234 336.107 (0.1000) 324 1.75×104
19 5 15 3 - 18 5 14 3 234 381.269 (0.1000) 316 1.80×104
19 7 13 4 - 18 7 12 4 234 441.264 (0.1000) 332 1.68×104
19 9 10 2 - 18 9 9 2 234 486.395 (0.1000) 166 1.51×104
19 9 11 0 - 18 9 10 0 234 502.241 (0.0009) 166 1.51×104
19 9 10 0 - 18 9 9 0 234 502.432 (0.0009) 166 1.51×104
19 9 11 1 - 18 9 10 1 234 508.614 (0.1000) 166 1.51×104
19 8 11 2 - 18 8 10 2 235 029.952 (0.1000) 155 1.61×104
19 8 12 0 - 18 8 11 0 235 046.493 (0.1000) 155 1.61×104
19 8 11 0 - 18 8 10 0 235 051.378 (0.1000) 155 1.61×104
19 8 12 1 - 18 8 11 1 235 051.378 (0.1000) 155 1.61×104
19 6 13 5 - 18 6 12 5 235 084.738 (0.1000) 324 1.76×104
20 3 18 4 - 19 3 17 4 235 200.422 (0.1000) 314 1.90×104
19 5 15 4 - 18 5 14 4 235 633.058 (0.1000) 316 1.81×104
19 7 13 0 - 18 7 12 0 235 844.544 (0.1000) 145 1.71×104
19 7 13 1 - 18 7 12 1 235 865.969 (0.1000) 145 1.67×104
20 2 18 3 - 19 2 17 3 235 904.655 (0.1000) 315 1.91×104
21 2 20 3 - 20 2 19 3 235 919.352 (0.1000) 319 1.94×104
19 7 12 0 - 18 7 11 0 235 932.379 (0.1000) 145 1.72×104
a-(CH2OH)262503 22 14 8 0 - 21 14 7 1 218 468.381 (0.0044) 221 1.51×104
CDMS 22 14 9 0 - 21 14 8 1 218 468.381 (0.0044) 221 1.51×104
20 4 16 1 - 19 4 15 0 219 764.925 (0.0040) 114 2.45×104
22 8 14 0 - 21 8 13 1 219 809.406 (0.0026) 156 2.24×104
28 12 16 0 - 28 11 17 0 233 426.949 (0.0036) 270 3.11×105
28 12 17 0 - 28 11 18 0 233 427.018 (0.0036) 270 3.10×105
28 12 16 1 - 28 11 17 1 233 451.651 (0.0036) 271 3.32×105
28 12 17 1 - 28 11 18 1 233 451.723 (0.0036) 271 3.32×105
22 5 18 1 - 21 5 17 0 233 536.696 (0.0025) 138 2.93×104
22 7 16 1 - 21 7 15 0 233 561.784 (0.0024) 149 2.79×104
22 6 17 1 - 21 6 16 0 234 264.446 (0.0026) 143 2.84×104
51 13 39 1 - 51 12 40 1 235 170.476 (0.0288) 737 4.73×105
23 3 21 1 - 22 3 20 0 235 170.573 (0.0023) 139 2.91×104
22 6 16 1 - 21 6 15 0 235 304.050 (0.0026) 143 2.90×104
14 5 9 1 - 13 4 10 1 235 326.413 (0.0021) 64 2.39×105
21 4 18 0 - 20 3 17 0 235 327.161 (0.0027) 122 6.30×105
23 2 21 1 - 22 2 20 0 235 600.178 (0.0022) 139 3.28×104
24 4 21 0 - 23 4 20 1 235 620.372 (0.0027) 155 2.88×104
26 1 26 0 - 25 1 25 1 235 834.239 (0.0040) 159 3.22×104
26 0 26 0 - 25 0 25 1 235 834.327 (0.0040) 159 3.22×104
Article number, page 28 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Table (C.1) Molecules and parameters used for the IRAS 16293B synthetic spectrum
Molecule Name Tag Database NTTex
(cm2) (K)
CCH Ethynyl radical 25501 CDMS 3.00×1013 120
HCN Hydrogen cyanide 27501 CDMS 5.00×1016 120
HNC Hydrogen isocyanide 27502 CDMS 5.00×1016 120
H13CN Hydrogen cyanide 28501 CDMS 2.00×1014 300
CO Carbon monoxide 28503 CDMS 1.00×1020 100
HC15N Hydrogen cyanide 28506 CDMS 2.00×1014 300
DNC Hydrogen cyanide 28508 CDMS 7.00×1014 300
13CO Carbon monoxide 29501 CDMS 3.10×1019 100
C17O Carbon monoxide 29503 CDMS 8.00×1016 100
H13C15 Hydrogen cyanide 29512 CDMS 2.00×1014 300
HNCH2Methanimine 29518 CDMS 8.00×1014 100
NO Nitrogen oxide 30008 JPL 2.00×1016 100
H2CO Formaldehyde 30501 CDMS 1.80×1018 105
C18O Carbon monoxide 30502 CDMS 1.00×1017 100
DCO+Formyl radical 30510 CDMS 3.00×1012 29
CH3NH2Methylamine 31008 JPL 5.30×1014 100
HDCO Formaldehyde 31501 CDMS 1.30×1017 105
H13
2CO Formaldehyde 31503 CDMS 3.60×1016 105
D2CO Formaldehyde 32502 CDMS 1.60×1016 105
H2C18O Formaldehyde 32503 CDMS 2.50×1015 105
CH3OH Methanol 32504 CDMS 2.00×1019 300
CH2DOH Methanol 33004 JPL 7.10×1017 300
13CH3OH Methanol 33502 CDMS 4.00×1016 300
NH2OH Hydroxylamine 33503 CDMS 3.70×1014 100
D13
2CO Formaldehyde 33506 CDMS 2.20×1014 105
HDC18O Formaldehyde 33510 CDMS 1.40×1014 105
H2S Hydrogen sulfide 34502 CDMS 1.00×1018 125
CH18
3OH Methanol 34504 CDMS 2.00×1016 300
HDS Hydrogen sulfide 35502 CDMS 2.00×1016 125
HD34S Hydrogen sulfide 37503 CDMS 1.00×1015 125
c-C3H2Cyclopropenylidene 38508 CDMS 2.00×1014 100
CH3CCH Propyne 40502 CDMS 6.80×1015 100
CH3CN Acetonitrile 41505 CDMS 4.00×1016 120
CH3CN ν8=1 Acetonitrile 41509 CDMS 4.00×1016 120
CH3NC Methyl isocyanide 41514 CDMS 2.00×1014 150
H2CCO Ketene 42501 CDMS 4.80×1016 125
HNCNH Carbodiimide 42506 CDMS 2.40×1016 300
13CH3CN Acetonitrile 42508 CDMS 3.30×1014 130
CH13
3CN Acetonitrile 42509 CDMS 3.00×1014 130
CH3C15N Acetonitrile 42510 CDMS 8.70×1013 130
CH2DCN Acetonitrile 42511 CDMS 5.60×1014 130
H2C13CO Ketene 43505 CDMS 7.10×1014 125
H13
2CCO Ketene 43506 CDMS 7.10×1014 125
HDC2O Ketene 43507 CDMS 2.00×1015 125
HNCO Isocyanic acid 43511 CDMS 3.70×1016 300
CHD2CN Acetonitrile 43514 CDMS 1.20×1014 130
H2N13CN Cyanamide 43515 CDMS 3.00×1013 300
N2O Nitrous oxide 44004 JPL 5.00×1016 100
DNCO Isocyanic acid 44006 JPL 3.00×1014 300
HN13CO Isocyanic acid 44008 JPL 4.00×1014 300
CS Carbon monosulfide 44501 CDMS 1.00×1016 125
c-C2H4O Ethylene oxide 44504 CDMS 4.10×1015 125
SiO Silicon monoxide 44505 CDMS 7.00×1013 300
s-H2CCHOH Vinylalcohol 44506 CDMS 1.20×1017 125
a-H2CCHOH Vinylalcohol 44507 CDMS 1.20×1017 125
C33S Carbon monosulfide 45502 CDMS 1.00×1014 125
Article number, page 29 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Table (C.1) Continued.
Molecule Name Tag Database NTTex
(cm2) (K)
NH2CHO Formamide 45512 CDMS 1.00×1016 300
CH3CDO Acetaldehyde 45524 CDMS 7.40×1015 125
CH2DCHO Acetaldehyde 45525 CDMS 6.20×1015 125
C34S Carbon monosulfide 46501 CDMS 3.00×1014 125
t-HCOOH Formic acid 46506 CDMS 5.08×1016 300
H2CS Thioformaldehyde 46509 CDMS 1.50×1015 125
NH13
2CHO Formamide 46512 CDMS 1.00×1014 300
CH3OCH3Dimethyl ether 46514 CDMS 3.00×1017 125
NH2CDO Formamide 46520 CDMS 1.40×1014 300
cis-NHDCHO Formamide 46521 CDMS 1.40×1014 300
trans-NHDCHO Formamide 46522 CDMS 1.20×1014 300
C2H5OH Ethanol 46524 CDMS 2.30×1017 300
HONO Nitrous acid 47007 JPL 9.00×1014 100
t-H13COOH Formic acid 47503 CDMS 8.30×1014 300
HDCS Thioformaldehyde 47504 CDMS 1.50×1014 125
CH13
3CH2OH Ethanol 47511 CDMS 4.60×1014 300
13CH3CH2OH Ethanol 47512 CDMS 4.60×1014 300
CH3CH2OD Ethanol 47515 CDMS 5.75×1014 300
CH3CHDOH Ethanol 47516 CDMS 1.15×1015 300
a-CH2DCH2OH Ethanol 47517 CDMS 1.34×1015 300
s-CH2DCH2OH Ethanol 47518 CDMS 6.51×1014 300
SO Sulfur monoxide 48501 CDMS 5.00×1014 125
C36S Carbon monosulfide 48503 CDMS 2.00×1013 125
CH3SH Methyl mercaptan 48510 CDMS 5.50×1015 125
CH35
3Cl Chloromethane 50007 JPL 3.10×1013 125
HC3N Cyanoacetylene 51501 CDMS 1.40×1014 100
CH37
3Cl Chloromethane 52009 JPL 2.20×1014 125
C2H3CN Vinyl cyanide 53515 CDMS 4.80×1014 110
C2H5CN Ethyl cyanide 55502 CDMS 1.50×1015 160
CH3NCO Methyl isocyanate 57505 CDMS 3.00×1015 300
CH3C(O)CH3Acetone 58003 JPL 3.40×1016 125
CH3CH2CHO Propanal 58505 CDMS 1.48×1015 125
CH3OCHO Methylformate 60003 JPL 2.60×1017 300
HOCH2CHO Glycolaldehyde 60501 CDMS 3.40×1016 300
OCS Carbonyl sulfide 60503 CDMS 2.00×1016 125
OCS ν2=1 Carbonyl sulfide 60504 CDMS 2.00×1017 125
CH3COOH Acetic acid 60523 CDMS 3.00×1015 300
O13CS Carbonyl sulfide 61502 CDMS 5.00×1015 125
OC33S Carbonyl sulfide 61503 CDMS 3.00×1015 100
HOCH13
2CHO Glycolaldehyde 61513 CDMS 4.46×1014 300
HO13CH2CHO Glycolaldehyde 61514 CDMS 4.46×1014 300
CH3O13CHO Glycolaldehyde 61515 CDMS 6.30×1015 300
DOCH2CHO Glycolaldehyde 61516 CDMS 4.86×1014 300
HOCHDCHO Glycolaldehyde 61517 CDMS 1.27×1015 300
HOCH2CDO Glycolaldehyde 61518 CDMS 6.25×1014 300
a-(CH2OH)2Ethylene glycol 62503 CDMS 1.37×1016 300
s-(CH2OH)2Ethylene glycol 62504 CDMS 3.62×1016 300
OC34S Carbonyl sulfide 62505 CDMS 1.50×1016 125
18OCS Carbonyl sulfide 62506 CDMS 7.00×1014 125
SO2Sulfur dioxide 64502 CDMS 1.50×1015 125
34O2Sulfur dioxide 66501 CDMS 4.00×1014 125
Article number, page 30 of 32
N.F.W. Ligterink et al.: The prebiotic molecular inventory of Serpens SMM1
Table (C.1) HOCH2CN lines in the PILS data towards IRAS 16293B
Transition Frequency Eup Aij Blending species
J,Ka,Kc,F(MHz) (K) s1
36 11 25 1 - 35 11 24 1 331 990.003 468 1.03×103g-(CH2OH)2
36 11 26 1 - 35 11 25 1 331 990.003 468 1.03×103g-(CH2OH)2
36 12 24 1 - 35 12 23 1 332 038.123 500 1.01×103CH3OCHO
36 12 25 1 - 35 12 24 1 332 038.123 500 1.01×103CH3OCHO
36 8 29 1 - 35 8 28 1 332 045.021 389 1.08×103g-(CH2OH)2
36 8 28 1 - 35 8 27 1 332 045.044 389 1.08×103g-(CH2OH)2
36 7 30 0 - 35 7 29 0 332 409.418 363 1.09×103
36 7 29 0 - 35 7 28 0 332 410.281 363 1.09×103
36 6 31 0 - 35 6 30 0 332 678.904 345 1.10×103
36 6 30 0 - 35 6 29 0 332 699.957 345 1.10×103
36 4 33 1 - 35 4 32 1 332 797.489 323 1.14×103
36 5 31 1 - 35 5 30 1 333 147.976 335 1.13×103
37 2 36 1 - 36 2 35 1 334 177.762 318 1.15×103CH3OCHO, CH3O13 CHO, C2H5CN, HO13CH2CHO
37 1 36 1 - 36 1 35 1 335 135.448 318 1.16×103CH3OH
37 1 36 0 - 36 1 35 0 335 187.943 313 1.16×103
36 4 32 1 - 35 4 31 1 335 446.009 324 1.17×103
36 2 34 1 - 35 2 33 1 335 757.704 310 1.15×103g-(CH2OH)2
36 4 32 0 - 35 4 31 0 335 825.850 318 1.15×103g-(CH2OH)2, CH3C(O)CH3, CH3OCHO
38 1 38 1 - 37 1 37 1 335 871.388 323 1.17×103
38 0 38 1 - 37 0 37 1 335 909.319 323 1.17×103
38 0 38 0 - 37 0 37 0 335 985.342 318 1.17×103CH3CH2OH, g-(CH2OH)2
36 3 33 0 - 35 3 32 0 338 263.867 311 1.07×103CH3OCHO
37 3 35 1 - 36 3 34 1 339 712.712 329 1.21×103NH2CHO
37 3 35 0 - 36 3 34 0 339 771.178 324 1.20×103
37 11 26 1 - 36 11 25 1 341 198.194 484 1.13×103
37 11 27 1 - 36 11 26 1 341 198.194 484 1.13×103
37 9 29 1 - 36 9 28 1 341 199.695 429 1.16×103g-(CH2OH)2
37 9 28 1 - 36 9 27 1 341 199.696 429 1.16×103g-(CH2OH)2
37 8 30 1 - 36 8 29 1 341 272.746 405 1.18×103HOCH2CHO, CH3CDO
37 8 29 1 - 36 8 28 1 341 272.781 405 1.18×103HOCH2CHO, CH3CDO
37 7 31 1 - 36 7 30 1 341 427.962 385 1.20×103CH3CHO
37 8 30 0 - 36 8 29 0 341 498.917 400 1.17×103CH3OCHO
37 8 29 0 - 36 8 28 0 341 498.954 400 1.17×103CH3OCHO
37 13 24 0 - 36 13 23 0 341 529.626 545 1.08×103a-(CH2OH)2
37 13 25 0 - 36 13 24 0 341 529.626 545 1.08×103a-(CH2OH)2
37 7 31 0 - 36 7 30 0 341 659.715 379 1.18×103
37 7 30 0 - 36 7 29 0 341 660.941 379 1.18×103
37 6 31 0 - 36 6 30 0 341 982.421 362 1.20×103CH3O13 CHO
37 4 34 1 - 36 4 33 1 342 001.917 339 1.23×103
37 5 33 1 - 36 5 32 1 342 118.267 352 1.23×103HOCH2CHO
37 5 32 1 - 36 5 31 1 342 514.548 352 1.23×103CH3CDO
38 2 37 1 - 37 2 36 1 342 945.040 335 1.25×103H2CS, CH3O13 CHO
38 2 37 0 - 37 2 36 0 343 045.762 330 1.24×103CH3CDO
39 0 39 1 - 38 0 38 1 344 625.126 339 1.27×103HONO, CH2DOH, CH3CHO
37 4 33 1 - 36 4 32 1 345 070.674 340 1.27×103CH3OCHO
37 4 33 0 - 36 4 32 0 345 471.229 335 1.25×103CH3OCHO
38 3 36 0 - 37 3 35 0 348 727.094 340 1.30×103
38 9 30 1 - 37 9 29 1 350 417.202 446 1.27×103a-(CH2OH)2
38 9 29 1 - 37 9 28 1 350 417.203 446 1.27×103a-(CH2OH)2
38 13 25 1 - 37 13 24 1 350 517.318 568 1.19×103
38 13 26 1 - 37 13 25 1 350 517.318 568 1.19×103
38 10 28 0 - 37 10 27 0 350 617.089 467 1.24×103CH3O13CHO
38 10 29 0 - 37 10 28 0 350 617.089 467 1.24×103CH3O13CHO
38 6 33 1 - 37 6 32 1 350 983.970 384 1.32×103CH3OCHO, CH3CDO
38 6 32 1 - 37 6 31 1 351 019.318 384 1.32×103H2C13 CO, CH3OCHO
38 6 32 0 - 37 6 31 0 351 269.282 378 1.30×103
38 5 34 1 - 37 5 33 1 351 402.459 369 1.33×103
Article number, page 31 of 32
A&A proofs: manuscript no. SMM1_C2H3NO_isomers
Table (C.1) Continued.
Transition Frequency Eup Aij Blending species
J,Ka,Kc,F(MHz) (K) s1
39 2 38 1 - 38 2 37 1 351 704.972 352 1.35×103
39 2 38 0 - 38 2 37 0 351 809.704 346 1.34×103
38 5 33 1 - 37 5 32 1 351 897.151 369 1.34×103
38 5 33 0 - 37 5 32 0 352 181.763 363 1.32×103CH3O13 CHO
39 1 38 0 - 38 1 37 0 352 488.531 346 1.35×103CH18
3OH
38 2 36 1 - 37 2 35 1 353 163.191 344 1.35×103
40 0 40 1 - 39 0 39 1 353 340.026 356 1.37×103CH3OCHO
40 1 40 0 - 39 1 39 0 353 395.992 351 1.37×103
40 0 40 0 - 39 0 39 0 353 420.250 351 1.37×103
38 4 34 0 - 37 4 33 0 355 133.419 352 1.36×103
39 3 37 1 - 38 3 36 1 358 294.680 363 1.33×103
39 9 31 1 - 38 9 30 1 359 634.337 463 1.37×103
39 9 30 1 - 38 9 29 1 359 634.339 463 1.37×103
39 12 27 0 - 38 12 26 0 359 881.373 545 1.30×103
39 12 28 0 - 38 12 27 0 359 881.373 545 1.30×103
39 7 33 1 - 38 7 32 1 359 919.218 419 1.41×103
39 13 26 0 - 38 13 25 0 359 947.354 579 1.28×103
39 13 27 0 - 38 13 26 0 359 947.354 579 1.28×103
39 8 32 0 - 38 8 31 0 359 967.849 434 1.38×103
39 8 31 0 - 38 8 30 0 359 967.931 434 1.38×103
39 16 23 1 - 38 16 22 1 360 033.494 705 1.21×103t-HCOOH
39 16 24 1 - 38 16 23 1 360 033.494 705 1.21×103t-HCOOH
39 14 25 0 - 38 14 24 0 360 034.284 617 1.25×103t-HCOOH
39 14 26 0 - 38 14 25 0 360 034.284 617 1.25×103t-HCOOH
39 6 33 1 - 38 6 32 1 360 303.101 401 1.43×103
39 4 36 1 - 38 4 35 1 360 369.224 373 1.45×103
40 2 39 0 - 39 2 38 0 360 565.572 364 1.44×103
39 5 35 1 - 38 5 34 1 360 684.867 386 1.44×103
40 1 39 1 - 39 1 38 1 361 063.161 369 1.46×103
39 5 34 1 - 38 5 33 1 361 297.474 386 1.45×103CH3CH2OH
39 5 34 0 - 38 5 33 0 361 594.894 381 1.43×103
39 2 37 1 - 38 2 36 1 361 805.086 361 1.46×103CH3OCHO
41 0 41 1 - 40 0 40 1 362 053.851 374 1.47×103CH3C(O)CH3
41 0 41 0 - 40 0 40 0 362 136.053 368 1.47×103
Notes. List of identified and unidentified HOCH2CN transitions with Aij 1.0×103s1towards IRAS 16293B in the PILS data set.
Article number, page 32 of 32
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Article
An experimental investigation of the photochemical and thermal evolution of ices deposited at 10 K which contain primarily H2O, CH3OH, NH3, and CO mixed in relative proportions consistent with the proposed composition of interstellar ices is presented. These experiments, which are relevant to both interstellar and cometary ices, are the first described in which CH3OH (methanol) is a major constituent of the ice. Ultraviolet photolysis of these ice analogs invariably produces H2CO, CO2, CO, CH4, and HCO, largely at the expense of photofragmented CH3OH. In addition, photolysis produces a mixture of more complex molecules, some of which contain nitrile or isonitrile (CN or CN) and carbonyl (CO) groups. Most of the CO and CH4 leaves the sample upon warm-up to 100 K. Most of the parent ice molecules sublime away by 200 K leaving behind a mixture of more refractory substances. Warm-up to 250 K removes a component rich in CH3 groups which may correlate with the carrier(s) of the CO and CN bonds. A residue rich in CH2 groups remains even after warm-up to 300 K. The relevance of these results to questions concerning the composition of interstellar and cometary ices, and the scale heights of photofragments in cometary comae is discussed. The results may have some bearing on the recent suggestion that polyoxymethylene is present in Comet Halley.
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